1.1 Technical Field
This invention pertains to concrete production, and in particular a method of cooling fine and coarse aggregates to lower the temperature of concrete.
1.2 Background Art
The production of concrete is typically accomplished by mixing cement, water, rock (coarse aggregate) and sand (fine aggregate). The particular ratio of water and cement is chosen to establish the strength and other properties desired for the specific use of each batch of concrete.
Weather conditions may indirectly affect the ultimate water-cement ratio, changing that ratio from pre-determined specifications. For example, if concrete is being mixed and poured in hot temperatures, more water may be required to hold slump constant. If the amount of water added at the job site significantly increases the water-cement ratio, the hardened concrete may have less strength, and be otherwise adversely affected.
Hot weather will increase the temperature of concrete components, resulting in increases to the temperature of the mixed and placed concrete. Warmer concrete causes the cement to react faster with the water, which causes a higher peak setting temperature. These increases to the temperature of the setting concrete result in expansion of the concrete. Eventually, the concrete will contract as it cools to ambient temperatures. This expansion and contraction weakens the concrete and causes thermal cracking.
To avoid the detrimental effects of hot weather on concrete production, a number of methods have been used to cool the concrete during mixing. For example, bagged, block, or flake ice has been used to lower the temperature of a concrete mixture. However, when using bagged and block ice as a cooling medium, it is difficult to control the total mix water content, and thus the water-cement ratio. Flake ice is associated with an expensive on-site ice production facility with ice weighing equipment. This system solves the water cement ratio control problem but at considerable expense.
Liquid nitrogen has also been used to cool concrete during mixing. For example, by injecting liquid nitrogen into ready-mix trucks at a job site, the concrete mixture can be reduced to a desired temperature immediately prior to pouring. Although this method of cooling is effective, the large amounts of liquid nitrogen required are costly.
Chilling concrete batch water with a mechanical refrigeration plant may be used to cool the concrete mixture, while still allowing the water-cement ratio to be exactly determined. However, since aggregate constitutes a much larger portion of the concrete mixture than the water component, the concrete mixture can be more effectively cooled by lowering the aggregate temperature in addition to chilling the water.
Coarse aggregate cooling has been successfully used to cool concrete production. An article written by the Applicant, "Economical Cooling of Hot Weather Concrete", Concrete Construction, September 1989, describes a method of cooling rock. In that method, the rocks which form the coarse aggregate component of a concrete mix may be cooled by evaporating water on the coarse aggregate surface. This is accomplished by keeping the coarse aggregates wet for a time period prior to batching by sprinkling or spraying ambient water onto the coarse aggregates. Although this method has limited usefulness in humid climates, coarse aggregate evaporative cooling in dry climates serves to reduce the aggregate temperature. For more significant temperature reductions, cold water may be used to inundate the coarse aggregates prior to concrete mixing. Coarse aggregate temperatures can be reduced to within 10 degrees Farenheit of the chilled water temperature using this method.
The novel process of this invention involves cooling the sand or rock component of concrete mixtures using chilled or refrigerated air, or other chilled or liquified gas, in a rotary drum device. Some methods of sand cooling are known in other industries. For example, in U.S. Pat. No. 4,304,286 to Waldron, sand is cooled as a part of a process of making moulds. The cooling process is accomplished by suspending the sand in a fluidized bed to which a blower supplies fluidizing air at ambient temperature, and then introducing a cryogenic liquid to contact the sand in its fluidized state.
A fluidized bed method of cooling sand for molding operations is also discussed in Nomura et al, "Continuous Cooling of Wet Foundry Sand Using a Fluidized Bed", Particulate Science and Technology 5:207-218, 1987. This article describes a theoretical model for cooling sand by evaporation, using ambient temperature air as the coolant air and water in a fluidized bed.
An apparatus for cooling foundry sand is described in U.S. Pat. No. 3,358,380 to Murphy. This device comprises a rotatable disc onto which sand is placed. The disc has perforations through which ambient temperature air is blown, to aerate and cool the sand.
U.S. Pat. No. 3,161,485 to Buhrer also discusses cooling sand for a molding process. Sand and cooling water are mixed with rotatable paddles in a trough. The sand exits the trough on a vibrating support, with perforations through which ambient temperature air is blown. Thus, the sand is cooled by evaporation as it travels on the vibrating support.
The methods known for cooling sand used in molding are useful for their intended purpose. However, the greater quantities of sand or rock required to produce concrete for large structures cannot efficiently be cooled in a fluidized bed or on a perforated carrier. The capital equipment costs would be prohibitively expensive. Similarly, the perforated carriers of Buhrer and Murphy are not well suited to quickly cooling vast quantities of sand or rock.
A process of cooling fine aggregates prior to concrete mixing is described in Goto et al., "Precooling Concrete Using Frozen Sand", Concrete International, 60-65, June 1990. Liquid nitrogen is used to reduce the temperature of the sand to below the freezing point of water, while the sand is stirred with mixing blades. While significant quantities of sand may be cooled with this process, the use of liquid nitrogen is very expensive.
A less expensive apparatus for cooling sand has been explored through mathematical models and small scale prototypes discussed in Riquelme and Navarro, "Analysis and Modeling of Rotatory Dryer--Drying of Copper Concentrate", Drying Solids, pgs 46-53, (John Wiley 1986), and Hirosue, "Influence of Particles Falling from Flights on Volumetric Heat Transfer Coefficient in Rotary Dryers and Coolers," Powder Technology, 59 (1989) 125-28. These articles teach the heating or drying of fine particles within a rotating drum. Ambient temperature air introduced into the drum effectuates a temperature change in the sand within the drum, although cooling is not an objective of the described processes.
A process of cooling large quantities of aggregates is needed, to efficiently and economically cool concrete production.